Advancing age, as well as injury, can lead to changes in the bones, discs, joints, and ligaments of the spine, producing pain from nerve compression. Under certain circumstances, alleviation of pain can be provided by performing spinal fusion. Spinal fusion is a procedure that generally involves the removal of the disc between two or more adjacent vertebrae and the subsequent joining of the vertebrae with a bone fixation device to facilitate growth of new osseous tissue between the vertebrae. The new osseous tissue fuses the joined vertebrae such that the vertebrae are no longer able to move relative to each other. Bone fixation devices can stabilize and align the injured bone segments to ensure the proper growth of the new osseous tissue between the damaged segments. Bone fixation devices are also useful for promoting proper healing of injured or damaged vertebral bone segments caused by trauma, tumor growth, or degenerative disc disease.
One such bone fixation device is a bone fixation plate that is used to stabilize, align, and, in some cases, immobilize adjacent skeletal parts such as bones. Typically, the fixation plate is a rigid metal or polymeric plate positioned to span bones or bone segments that require stabilization, alignment, and/or immobilization with respect to one another. The plate may be fastened to the respective bones, usually with bone screws, so that the plate remains in contact with the bones and fixes them in a desired position. Bone plates can be useful in providing the mechanical support necessary to keep vertebrae in proper position and bridge a weakened or diseased area such as when a disc, vertebral body or fragment has been removed or during spinal fusion.
Such plates have been used to stabilize, align, and/or immobilize a variety of bones, including vertebrae of the spine. For example, a bone plate may include a plurality of screw openings, such as holes or slots, for screw placement. The bone plate may be placed against the damaged vertebrae and bone screws or other bone anchors can be used to secure the bone plate to the vertebrae. In the case of spinal fusion, for example, a prosthetic implant or bone graft may be positioned between the adjacent vertebrae to promote growth of osseous tissue and fusion of the vertebrae.
In spinal fusion procedures, conventional bone plates generally immobilize the connected vertebrae, imposing a rigid compressive load on the vertebrae. Gaps may develop in the new osseous tissue growing between the vertebrae, which can result in decoupling of the compressive load on the osseous tissue and the implant or graft positioned between the vertebrae. To address this problem, dynamic plates have been proposed that aim to permit the vertebrae to collapse axially during fusion. Examples of such dynamics plates are disclosed in commonly owned U.S. patent application Ser. No. 10/664,238, filed Sep. 17, 2003, entitled “Bone Fixation Plates,” which is hereby incorporated by reference. Such exemplary dynamic plates are preferably implanted in the fully extended state. Manipulation of the plate during implantation can be challenging, as the segments of the plate may move relative to one another. There is a need for an instrument to assist in implanting dynamic plates in an extended state